Optical sensors have significant advantages over electronic sensors in many applications; for instance, those requiring absolute explosion proofing, insensitivity to electromagnetic interference (EMI), low to zero electronic emissions, high voltage exposure during electrical current measurements, and/or complete absence of metals or other electrical conductors. Sensors connected to and/or interrogated through optical fibers (fiber optic sensors; or FO sensors) are being rapidly developed, although the industry is still in its infancy.
One type of FO sensor that has been difficult to reduce to a practical industrial product is a magneto-optical fiber optic sensor (MOFO). This is perhaps partly because of the lack of magnetic sensitivity in most optical materials, lack of the proper form of materials that do have high sensitivity and partly because of the polarization-dependent nature of the sensors that can be made from existing materials.
The illustrative exemplary non-limiting technology herein provides MOFO sensors and methods of manufacture that substantially eliminate the uncertainties in the magnetic field signals due to polarization dependence, provide high signal-to-noise ratios (SNR) and replace the light intensity (amplitude) dependence of prior art MO sensors with signals based on spectral changes in the interrogating light wavelength band.
Prior art MO sensors have been based on the rotation of light polarization during transmission through a material with a high Faraday Effect (e.g., Bi-doped, yttrium-iron garnets, or Bi:YIG) (see, for example Rochford K. B., Rose A. H., Deeter M. N., Day G. W. “Faraday effect current sensor with improved sensitivity-bandwidth product.” Optics Letters, vol. 19, (no. 22), November 1994, p. 1903 and Day G. W., Deeter M. N., Rose A. H.; Rochford K. B. “Faraday effect sensors for magnetic field and electric current.” Proceedings of the SPIE—The International Society for Optical Engineering, vol. 2292, (Fiber Optic and Laser Sensors XII, San Diego, Calif., USA, 25-27 Jul. 1994.) 1994, p. 42), a high Verdet constant (rare-earth doped silica glass) (see, e.g., Y. N. Ning et al in “Recent progress in optical current sensing techniques”, Review of Scientific Instruments 66 (5), May 1995, pp. 3097-3111), long lengths of low Verdet constant glass fiber (lengths of a kilometer or more) (see for example [Rose A. H. “Playing with fire and fibers”, IEEE Circuits and Devices Magazine, vol. 15, (no. 5), IEEE, September 1999. p. 41]) or reflection from a material with a substantial Kerr effect (ferromagnetic metals) [Oliver, S. A. and C. A. DiMarzio, U.S. Pat. No. 5,736,856; U.S. Pat. No. 5,631,559; U.S. Pat. No. 5,493,220]. In the scheme attributed to Malus, the magnetic field signal from each of these sensors, whether connected by optical fiber or not, is an amplitude signal because crossed polarizers or similar techniques are used to detect the polarization rotation. The light rotated by an external magnetic field measurand is detected as a varying light intensity, or amplitude. The signals are thus vulnerable to amplitude variability and interference due to fiber bending losses, dirty connectors and other similar causes, and large inaccuracies can occur. Polarization-maintaining (PM) fiber can be used to avoid some of the errors due to spurious polarization rotation in the fiber, but cannot solve other amplitude losses. The alternative measurement technique, which employs Sagnac interferometers (J. Blake et al., IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996; Moon Fuk Chan; Guansan Chen; Demokan M. S.; Hwa Yaw Tam “Optimal sensing of current based on an extrinsic Sagnac interferometer configuration”, Optics and Lasers in Engineering, vol. 30, (no. 1), July 1998. p. 17), relies on phase measurements but is also vulnerable to amplitude and polarization variability, although not at such a high degree as the Malus scheme.
Another difficulty with prior art Faraday Effect sensors, excluding those of the type based on kilometer-length fiber coils, is that the sensors utilize either relatively large blocks of high Verdet constant glass (several centimeters) or small blocks of a high Faraday Effect material such as perpendicular magnetization Bi:YIG. Here, perpendicular magnetization is taken to mean that the magnetization vector in the material is perpendicular to the wafer plane, with no external magnetic field present. This orientation of the magnetization can be arranged if the wafer is cut from a YIG crystal boule, or if a thick YIG film is grown on a substrate wafer, such as is done with liquid phase epitaxy (LPE) on a gadolinium gallium garnet (GGG) substrate, or on an expanded lattice GGG variation doped with materials such as calcium, magnesium, zirconium or scandium. In these cases, the transmission of light is parallel to the magnetization vector and the magnetic field is detected perpendicular to the film plane. This is because the magnetization vector is constrained from rotating due to the shape anisotropy of the material block, preventing any signal from being caused by magnetic fields perpendicular to the light path. In particular with the YIG Faraday Effect sensors, the sensor may be made up of several components (AIRAK, Inc. U.S. Pat. No. 6,534,977) and then glued or otherwise mounted to the fiber, making a fragile and unreliable sensor.
Still further difficulty is found with prior art magnetic field sensors if electrical current is to be measured. With kilometer-length, low Verdet constant fibers, coils are made and the wire is passed through the center. With high voltage power cables, the only way this can be done conveniently is to deliver the sensor with a de-mountable buss bar system—a very costly method. The same is substantially true of high Verdet constant glass sensors, since the blocks of glass must be fashioned into prisms and placed around the buss bar in a robust holding assembly so the optical path circumvents the buss bar—also a very costly method. On the other hand, the YIG sensors such as those disclosed in U.S. Pat. No. 6,534,977 are essentially “point” sensors, but suffer from the need to assemble them from many components that must be kept clean and aligned in their package. This is in addition to the difficulties with amplitude, polarization and sensor robustness discussed above.
The illustrative exemplary non-limiting implementations herein provide sensors based on planar optical waveguide resonators with waveguide modes at least partially localized in the one or more layers of materials exhibiting the Faraday Effect (i.e. magneto-optical materials). The magnetization in such magneto-optical materials is preferably the in-plane type of magnetization, i.e., materials in which the magnetization vector lies in, or nearly in, the plane of the wafer from which the sensor is fashioned. Further, the optical waveguide(s) lies in the plane of the wafer as well.
Exemplary illustrative non-limiting implementations also provide MOFO sensors in which the Faraday Effect material can remain a simple plane-parallel disk shape with no cuts or corners, allowing the magnetization vector to rotate freely in the plane of the sensor disk, greatly increasing the measurement sensitivity and resolution.
Exemplary illustrative non-limiting implementations can provide MOFO sensors with very few parts (the planar chip, the fiber connections and the package) such that the sensor is robust and stable.
Exemplary illustrative non-limiting implementations may also provide MOFO sensors that convert the external field magnitude to an optical wavelength shift that is accurately and unambiguously detectable. Further, crossed polarizers are not required, and the polarization sensitivity is reduced to at least the second order, allowing polarization-maintaining fiber connections to eliminate over-all polarization dependence.
Exemplary illustrative non-limiting implementations further provide effective “point” magneto-optical sensors for the detection of magnetic fields and magnetic field gradients due to electrical currents, permanent magnets, the earth's magnetic field or any other source.